The experimental objectives of this proposal focus on the elucidation of the structural factors which regulate the interactions between physiological electron transport proteins, the regulation of prosthetic group potential in such proteins, and the precise mechanism by which these proteins catalyze efficient electron transfer to or from physiological oxido-reductants. A mechanistic understanding of these phenomena is essential to the eventual elucidation of the mode of regulation of electron transport in phosphorylating electron transport chains and the fundamental mechanism by which electron transfer between "non-isopotential" physiological redox components is coupled to energy conservation. The electron transport proteins whose x-ray structure determinations are proposed here each possess unique structural and functional characteristics which bear directly upon the problems of structure/function interrelationship and mechanisms outlined above. The specific proteins whose structure determination is proposed are Chl. thiosulfatophilum cytochrome c555, Rps. Palustris cytochrome c', and Rps. capsulata ytochrome c'.